We recently discovered that cytochrome c (Cytc) is directly regulated by cell signaling via tyrosine phosphorylation on two distinct sites in mammalian liver and heart, a finding made possible by isolation techniques developed by us that preserve the physiological phosphorylation status of the protein. It is our overall hypothesis that Cytc phosphorylation regulates the two main functions of Cytc, mitochondrial respiration and Cytc release during apoptosis. Our long term goal is to understand the effect of Cytc phosphorylation on respiration and apoptosis and to identify the cell signaling pathways directed to Cytc. Supported by preliminary data, three specific aims will be investigated: 1) to test the hypotheses that Cytc is phosphorylated in vivo in a distinct tissue-specific manner and that this phosphorylation results in structural changes of Cytc; 2) to test the hypotheses that phosphorylation of Cytc leads to controlled respiration and prevents its participation in apoptosis; and 3) to systematically identify mitochondrial tyrosine kinases that phosphorylate Cytc. Phosphorylated Cytc will be isolated from cow liver and heart tissue and structurally characterized using mass spectrometry and protein crystallography (Aim 1). Phosphorylated Cytc will be subjected to functional assays including in vitro respiration and apoptosis measurements, accompanied by mutagenesis studies in vivo with Cytc constructs in a Cytc knockout cell line (Aim 2). Experiments to identify kinases that act on Cytc will include a proteomic approach, kinase localization, and yeast two-hybrid studies (Aim 3). This research is expected to reveal that Cytc, long recognized as a central molecule in respiration and apoptosis, is subject to regulation by cell signaling, opening new opportunities for the understanding and control of those two key biological processes.